1. Field of the Invention
The present invention relates to a semiconductor device having an active layer of a semiconductor thin film formed on a substrate having an insulating surface, and particularly to a thin film transistor in which an active layer is made of a crystalline silicon film.
2. Description of the Related Art
In recent years, attention has been paid to a technique in which a thin film transistor (TFT) is constituted by using a semiconductor thin film (thickness of about hundreds to thousands xc3x85) formed on a substrate having an insulating surface. The thin film transistor is widely applied to an electronic device such as an IC or an electro-optical device, and especially, its development as a switching element for an image display device has been hurried.
For example, in a liquid crystal display device, attempts have been made to apply TFTs to any electric circuit such as a pixel matrix circuit for respectively controlling pixel regions arranged in a matrix form, a drive circuit for controlling the pixel matrix circuit, and a logic circuit (processor circuit, memory circuit, etc.) for processing data signals from the outside.
In the present circumstances, although a TFT using an amorphous silicon film as an active layer is put into practical use, an electric circuit required to have further high speed operational performance, such as a drive circuit and a logic circuit, demands a TFT using a crystalline silicon film (polysilicon film).
As a method of forming a crystalline silicon film on a substrate, techniques disclosed in Japanese Patent Unexamined Publication No. Hei 6-232059 and No. Hei. 6-244103 by the present applicant are well known. The techniques disclosed in these-publications enable the formation of a crystalline silicon film having excellent crystallinity by using a metal element (especially nickel) for promoting crystallization of silicon and by a heat treatment at 500 to 600xc2x0 C. for about four hours.
Japanese Patent Unexamined Publication No. Hei. 7-321339 discloses a technique for carrying out crystal growth substantially parallel to a substrate by utilizing the above techniques. The present inventors refer to the formed crystallized region as especially a side growth region (or lateral growth region).
However, even if a drive circuit is constituted by using such a TFT, the drive circuit is still far from the state of completely satisfying the required performance. In the present circumstances, especially it is impossible to constitute a high speed logic circuit requiring electric characteristics of extremely high performance to realize both high speed operation and high withstand voltage characteristics at the same time, by a conventional TFT.
As described above, in order to attain the higher performance of an electro-optical device and the like, it is necessary to realize a TFT having performance comparable with a MOSFET formed by using a single crystal silicon wafer.
An object of the present invention is therefore to provide a thin film semiconductor device having extremely high performance as a breakthrough for realizing higher performance of an electro-optical device, and a method of manufacturing the same.
It is conceivable, as a reason why a high performance TFT as mentioned above has not been able to be obtained by a conventional method, that carriers (electrons or holes) are captured by crystal grain boundaries so that improvement of an field effect mobility as one of parameters showing TFT characteristics has been prevented.
For example, there are many unpaired bonds (dangling bonds) of silicon atoms and defect (capture) levels in the crystal grain boundaries. Accordingly, since carries moving in the inside of each crystal are easily trapped by the dangling bonds, defect levels or the like when they come close to or come into contact with the crystal grain boundaries, it is conceivable that the crystal grain boundaries have functioned as xe2x80x9cmalignant crystal grain boundariesxe2x80x9d to block the movement of the carries.
In order to realize a semiconductor device of the present invention, it is indispensable to provide a technique to change the structure of such xe2x80x9cmalignant crystal grain boundariesxe2x80x9d into xe2x80x9cbenign crystal grain boundariesxe2x80x9d for carriers. That is, it is important to form crystal grain boundaries which have a low probability of capturing carriers, that is, a low possibility of blocking the movement of carriers.
Therefore, according to the present invention disclosed in the present specification, a method of manufacturing a semiconductor device including an active layer of a semiconductor thin film, comprises the steps of forming an amorphous silicon film on a substrate having an insulating surface, forming a mask insulating film selectively on the amorphous silicon film, making the amorphous silicon film selectively hold a metal element for promoting crystallization, transforming at least a part of the amorphous silicon film into a crystalline silicon film by a first heat treatment, removing the mask insulating film, forming an active layer made of only the crystalline silicon film by patterning, forming a gate insulating film on the active layer, carrying out a second heat treatment in an atmosphere containing a halogen element so that the metal element in the active layer is removed through gettering and a thermal oxidation film is formed in an interface between the active layer and the gate insulating film, and carrying out a third heat treatment in a nitrogen atmosphere to improve film qualities of the gate insulating film including the thermal oxidation film and the state of the interface, wherein the active layer is a crystalline structure body in which crystal grain boundaries are aligned substantially in one direction and which is constituted by an aggregation of a plurality of needle-shaped or column-shaped crystals substantially parallel with the substrate.
If a crystalline silicon film is formed in accordance with the above manufacturing method, a thin film having an appearance as shown in FIG. 9 is obtained. FIG. 9 is an enlarged micrograph of the thin film in the case where the present invention was practiced by using the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 7-321339 as means-for crystallizing an amorphous silicon film, and shows a lateral growth region 901 having a length of several tens to a hundred and several tens xcexcm.
The lateral growth region 901 has a feature that since the needle-shaped or column-shaped crystals grow almost vertically to a region (designated by 902) in which a metal element for promoting the crystallization has been added, and substantially parallel with each other, the directions of crystals are aligned. A portion designated by 903 is a macroscopic crystal grain boundary (differentiated from crystal grain boundaries between needle-shaped crystal and column-shaped crystals) formed by collision between needle-shaped crystal and column-shaped crystals extending from the opposing added regions 902.
FIG. 10 is a TEM photograph in which a minute region of the inside of a crystalline grain is further enlarged with paying attention to the inside of the lateral growth region shown in FIG. 9.
That is, although the crystalline silicon film of the present invention seems to be macroscopically composed of the large lateral growth region 901 as shown in FIG. 9, when the lateral growth region 901 is microscopically observed, the lateral growth region is such a crystalline structure body as to be constituted by a plurality of needle-shaped or column-shaped crystals 1001 as shown in FIG. 10.
In FIG. 10, reference numeral 1002 denotes a crystal grain boundary showing a boundary between the needle-shaped or column-shaped crystals, and from the direction of extension of the crystal grain boundary 1002, it is confirmed that the needle-shaped or column-shaped crystals 1001 grew substantially parallel to each other. Incidentally, the crystal grain boundary in the present specification indicates a boundary between the needle-shaped or column-shaped crystals unless specified otherwise.
In the semiconductor device of the present invention, the metal element (mainly nickel) for promoting crystallization is removed through gettering by the heat treatment in the atmosphere containing a halogen element, so that the concentration of nickel, which has remained at a concentration of not less than 1xc3x971018 atoms/cm3, is reduced to not larger than 1xc3x971018 atoms/cm3, typically to 1xc3x971014xe2x88x925xc3x971017 atoms/cm3 (preferably not larger than spin density in the active layer).
Of course, it is conceivable that other metal element (Cu, Al etc.) mixed by contamination or the like (not added intentionally) is similarly removed through gettering.
At this time, it is expected that dangling bonds of silicon atoms are combined with oxygen during the heat treatment to form oxide (silicon oxide). As a result, the silicon oxide is formed in the region of xe2x80x9cmalignant crystal grain boundariesxe2x80x9d, and it is conceivable that the silicon oxide substantially functions as crystal grain boundaries.
It is inferred that the crystal grain boundary 1002 formed in this way has a state in which lattice defects are hardly included in the interface between the silicon oxide and crystalline silicon so that the alignment is excellent. This is because silicon atoms between lattices which cause defects are consumed by the synergy effect of a process in which the silicon oxide is formed by thermal oxidation and a process in which recombination between silicon atoms themselves or silicon atoms and oxygen atoms is promoted by the catalysis of nickel.
That is, in FIG. 10, it is conceivable that the crystal grain boundary 1002 has little defects to capture carriers so that it behaves as xe2x80x9cbenign crystal grain boundaryxe2x80x9d which functions as only an energy barrier for carriers moving in the inside of the needle-shaped or column-shaped crystal.
Since thermal oxidation reaction proceeds with priority in such a crystal grain boundary, a thicker thermal oxidation film is formed in the crystal grain boundary than other regions. Thus, it is inferred that a gate voltage applied to the vicinity of the crystal grain boundary becomes apparently small, which also can become an energy barrier.
Further, since this heat treatment is carried out at a relatively high temperature exceeding 700xc2x0 C. (typically 800-1100xc2x0 C.), crystal defects such as dislocation or stacking fault existing in the inside of the needle-shaped or column-shaped crystal are almost vanished. Furthermore, the remaining dangling bonds of silicon atoms are terminated by hydrogen or a halogen element contained in the film.
Accordingly, the present inventors define, in the state obtained in this way and shown in FIG. 10, the region of the inside of the plurality of needle-shaped or column-shaped crystals as xe2x80x9cregion considered to be substantially single crystal for carriersxe2x80x9d.
The feature xe2x80x9cconsidered to be substantially single crystal for carriersxe2x80x9d means that there is no barrier to block the movement of carriers when the carriers move. In other words, there are no crystal defects and no grain boundaries, or no potential barriers as energy barriers.
The present invention provides a semiconductor device with high performance which can constitute a drive circuit or a logic circuit by using a crystalline silicon film having the structure as described above so as to form an active layer of the semiconductor device typified by a TFT.